In the case of the long-term storage of engines, involved preservation work and regular maintenance is required in order to avoid corrosion of bearings and gears, contamination and aging of the operating fluids and to keep the seals from drying out. The high expense for personnel and costs incurred during the preservation and maintenance work is particularly glaring in the case of unmanned aerial vehicles (UAVs), which are often stored for up to 10 years without being deployed.
In the case of the separate long-term storage of engines, relevant maintenance instructions require operating fluids such as e.g., the fuel and lubricating oil to be replaced with an appropriate preservation oil. In this case, regular cranking of the stored engine is prescribed in order to wet the bearings, gear wheels and pumps or to keep the seals moist etc. with the preservation oil. Cranking is traditionally carried out by hand, which, in turn, is reflected by the high personnel costs.
Instead of this, engines installed in an aircraft must in some cases be cranked at regular intervals by a starter and pass ground runs. In this case, the fuel consumed in particular during the ground runs as well as the required safety measures result in high costs.
In addition, the preservation oil for separate long-term storage in a container as well as for long-term storage in the aircraft has to be changed after a prescribed storage time.
FIG. 1 depicts a fuel system and oil system that must be maintained in a conventional manner. The fuel system 2 has a fuel tank 6 to accommodate a fuel 8, which is conveyed via a filling pump 10 to the engine fuel pump 16. The fuel tank 6 has a tank vent 12 and an outlet line 14 for removing the fuel 8. The outlet line 14 is connected to a forepump 16 and has a shut-off valve or a main valve 18 to control the outlet line 14 to open or close, which is configured as a 2/2 directional valve. In its first operating position 1, which represents the fail-safe mode, the main valve 18 is controlled to close and no fuel 8 may be removed from the fuel tank 6. In its depicted second operating position 2, the main valve 18 is controlled to open and fuel 6 may be removed from the fuel tank 8.
The forepump 16 is connected to a gear mechanism 20 and is connected to a fuel line 22, which discharges into a fuel distribution system 24 of a combustion chamber 26. To prevent impurities of the fuel 8 in the fuel line 22, a filter 28 is arranged in said line. A gear pump 30, which can be triggered via the gear mechanism 20, is provided downstream from the filter 28. To meter out the fuel quantity which is supplied or is supposed to be supplied to the distribution system 24, a fuel metering device 32 with an overflow line 34 is connected downstream from the gear pump 30. The overflow line 34 discharges upstream from the filter 28 into the fuel line 22 and makes it possible to recirculate an excess fuel quantity. Upstream from the fuel metering device 32, one or more fuel-hydraulic actuators 36 with electro-hydraulic servo valves 38 are connected to the fuel line 22 via one or more branch lines 40. The fuel-hydraulic actuator 36 has a piston 42, which is movably guided in a cylinder 44. The fuel 8 may be recirculated into the fuel line 22 via an actuator line 46, which extends from the electro-hydraulic servo valve 38 and discharges upstream from the filter 28 into the fuel line 22. To increase a fuel pressure for the actuator 36, the servo valve 38, and other valves integrated into the fuel metering device 32, such as a metering valve, engine shut-off valve and engine emergency shut-off valve, a pressure-increasing valve 48 is connected downstream from the fuel metering device 32, which pressure-increasing valve is operatively connected to the actuator line 46 via a control line 50. To generate a heat exchange between the fuel 8 and an oil 56 conveyed in the oil system 4, a heat exchanger 52 is arranged in the fuel line 22 between the forepump 20 and the filter 28. The heat exchanger 52 may also be arranged in another position, for example behind the pressure-increasing valve 48.
Described in a very simplified manner, the oil system 4 has an oil tank 54 for accommodating the oil 56, an outlet channel 58 for removing the oil 56 as well as a filling channel 60 for filling the tank. In addition, the oil tank 54 has an overflow channel 62, a drainage channel 64, for example for sampling, a tank ventilation channel 66 and an oil level sensor 68. The outlet channel 58 is connected to a feed pump 70, which is operatively connected to the gear mechanism 20. Extending from the feed pump 70 are at least one oil channel 72 to bearing compartments and gear mechanisms 74 of the engine as well as to a starting unit or generator 76. The starting unit or the generator 76, however, does not necessarily represent a part of the oil system 4, but may have a separate oil system. The oil channel 72 is guided through the heat exchanger 52 and has a filter upstream from the heat exchanger 52 to prevent impurities. The bearings and gear wheels in the bearing compartments and gear mechanisms 74 as well as the generator 76 respectively have a suction channel 80, 82, which leads respectively to the oil tank 54 and in which a corresponding suction pump 84 is respectively arranged. In addition, the bearing compartments and gear mechanism 74 as well as the starting unit 76 and the oil tank 54 have ventilation channels 86, 88, 66 to discharge oil, bearing compartment sealing air, which, for example, in the case of higher pressure, guarantees a leak tightness of e.g., gap seals, brush seals, labyrinth seals or mechanical seals 96, 98 between bearing compartments and main shafts, and other gas vapors, which are in fluid connection with an centrifugal oil separator 90. The oil separator 90, which has an operative mechanical connection to the gear mechanism 20, has a duct 92 to recirculate the oil 56 deposited in the oil separating device 90 to the oil tank 54 as well as a ventilation channel 94 to release the bearing compartment sealing air and gas vapors. To prevent air from penetrating into the oil system 4 via the bearing compartments and gear mechanism 74 as well as the starting unit 76, these are virtually hermetically sealed via corresponding seals 96, 98. Said seals are under pressure with sealing air from outside the air chambers 74. The so induced air flow in the chambers 74 prevents oil 56 from escaping through the seals 96, 98.
In the case of the known oil and fuel systems described above, the pumps 16, 30, 70, 84 as well as the oil separator 90 are triggered respectively by a gear mechanism 20, which is operatively connected to an engine shaft, in particular a high-pressure shaft of the engine. Similarly, the electric generator/starter 76 is driven via a or the gear mechanism 20 and therefore via the engine shaft. Consequently, the fuel pumps 16, 30 as well as the oil pumps 70, 84 possess a speed which is proportional to the rotational speed of the engine shaft. In the optional function as a starter, the generator/starter 76 puts the engine shaft into rotation via the gear mechanism 20. Instead of the electric starter 76, an air turbine motor (not shown) is frequently used for starting.
FIG. 2 depicts a recent concept of an oil system and a fuel system. In this regard, see the applicant's German Patent Application DE 10 2007 051 498 A1. In contrast to the oil system 4 and fuel system 2 shown in FIG. 1, these more recent systems 2, 4 each have an electric motor 102, 104 for driving the fuel pumps 16, 100 or the oil pumps 70, 84 as well as the oil separation device 90. In this case, on the fuel side, the previous gear pump 30 located downstream is designed here as an adjustable displacement pump 100. In addition, the conventional fuel metering device 32 is replaced with the pump 100 and a fuel through-put measuring device 106 downstream from the pressure-increasing valve 48 for increasing the fuel pressure. Because of the adjustability of the displacement pump 100, it is possible to dispense with an overflow line 34. The internal or external starting unit 76 is driven via a shaft of the engine with the interconnection of a gear mechanism. As an option, the high-pressure shaft and the low-pressure shaft of the engine may be provided with directly driven forward or rear generators or starter/generators. This recent concept permits variable and individual adjustment of the feed rates of the oil and fuel pumps 70, 84, 16, 100. However, the fact that this concept also entails substantial maintenance cost is problematic, because, for example, fluid exchange of the fuel system in particular is expensive and the bearings still need to be wetted with oil regularly and cranked as described at the outset. Similarly, monitoring the systems and particularly the quality of the preservation fluid is expensive.